Interface-powered converters.

Interface-powered converters

Interface-powered converters plug directly into RS-232 devices, eliminating the need for external power supplies. These converters draw their power from the connected RS-232 device’s signals, normally the Transmit Data (TD), Request to Send (RTS), and Data Terminal Ready (DTR) pins.

A few things to keep in mind

When using an interface-powered converter, there are a few things to keep in mind, such as how much power is available from the RS-232 port—and how much power is dissipated in the system.

The quality and length of the cable used on both the RS-232 and the RS-422/485 side can affect how power is dissipated. Because lower-quality and higher-gauge cable can cause power loss, a good-quality 24 AWG cable is recommended. Also RS-232 runs longer than 6 feet (1.8 m) and RS-422 runs longer than the 4000-foot (1219.2-m) standard can drain cable. You might even want to consider shorter runs for the RS-422/485 cable side. The tests shown in the chart (right) were all performed with cable lengths of 1000 feet (304.8 m).

In addition, the data rate can also affect the dissipation of power along the line. The most common data rate for this type of equipment is 19.2 kbps.

Termination resistors are not recommended with interface-powered converters. They add heavy DC loading to the system and may overload the converters.

Also, it’s best not to make loopback connections because they dissipate a lot of power.

Plus, because the RS-232 port can only supply limited power, you must consider the types of interface-driver integrated circuits used and the state of the output lines. The ability to deliver power is greater if the output is in the positive voltage state.

The chart below compares several types of driver-integrated circuits.

Comparison of RS-232 Drivers

Type of
RS-232 Driver

State of Line

Voltage at
+/- Condition

Voltage/Current per Driver (Max.)

Power Available (from 3 Drivers)

1488

Positive

+9 V/+11 V

+5V @ 6.5 mA

98 mW

1488

Negative

-9 V/-11 V

-5V @ 6.5 mA

78 mW*

MAX232
Charge Pump

Positive

+7.5 V/+8 V

+5V @ 5 mA

75 mW

MAX232
Charge Pump

Negative

-7.5 V/-8 V

-5V @ 3.5 mA

42 mW*

EIA-562

Positive

+3.7 V/+5 V

+4.5V @ 1.5 mA

—

EIA-562

Negative

-3.7 V/-5 V

-4.5V @ 1.5 mA

—

*NOTE: The voltage inverter in the interface-powered converter dissipates power when converting negative voltage.